The invention relates to forming a pn junction in a semiconductor structure and, in particular, to forming a hyper-abrupt pn junction by low-temperature growth of a doped film on a surface of an oppositely doped substrate wherein the metallurgical junction of the pn junction could coincide with the surface of the substrate.
The pn junction is the basic building block of most semiconductor devices. It is possible to fabricate these structures by a number of processes that are familiar to those working in the semiconductor industry. Briefly, such techniques used to produce pn junctions include thermal diffusion, ion implantation and epitaxial growth. The present invention is directed to pn junctions formed by growth of epitaxial layers taking the form of any one of silicon (Si), germanium (Ge), silicon germanium (SixGe1xe2x88x92x), or silicon germanium carbide (SixGeyC1xe2x88x92xxe2x88x92y), which are used extensively in the production of silicon-based microelectronic devices.
A Si epitaxial layer is typically deposited, or grown, on a Si substrate by high temperature pyrolysis of a chlorosilane precursor, such as dichlorosilane, or at a low temperature if silane (SiH4) is the precursor. Another approach is to deposit this layer by molecular beam epitaxy (MBE). MBE produces well behaved, i.e., abrupt, pn junctions, but the practicality of MBE is limited due to its extreme vacuum requirements and very low growth rates.
A commercially viable alternative to MBE is Plasma-Enhanced Chemical Vapor Deposition (PECVD), which has the potential to reduce the substrate temperature required for growing an epitaxial layer for forming a pn junction. To date, however, it is believed PECVD has not produced a properly operating pn junction in which the initial growth surface is contained in the depletion layer or located at the metallurgical junction of the oppositely-doped materials and the doping concentration of the least doped side is greater than 1016 cmxe2x88x923.
In general, the gaseous discharge associated with PECVD assists in creating the precursors to film growth, such as SiH, SiH2 and SiH3, and also assists in removing impediments to film growth, e.g., hydrogen on the substrate surface. The movement of chemical species on the growth surface is enhanced by the energetic particle flux to the surface, which in general assists in the overall growth process. It is known that growing epitaxial layers at lower substrate temperatures reduces unwanted autodoping, dopant diffusion and the creation of crystal defects. The ability to epitaxially deposit Si-based and Ge-based films at reduced substrate temperatures would improve the quality of semiconductor devices, would simplify the process used to fabricate such devices and would make possible the fabrication of new device structures. Implicit in this capability to fabricate devices is the ability to chemically alter or dope the material with impurity atoms of n or p type. The ability to spatially alternate the placement of these dopant atoms in adjacent regions is required for the construction of a pn junction. There is considerable benefit to the ability to incorporate these dopants at low substrate temperatures that reduce unwanted autodoping and dopant diffusion that degrade the abruptness of a pn junction.
Currently there is considerable interest in growing SixGe1xe2x88x92x and SixGeyC1xe2x88x92xxe2x88x92y alloys for producing heterojunction devices. Successful epitaxial growth of these heteroepitaxial alloys on Si substrates requires that low substrate temperatures (i.e., below 650xc2x0 C.) be used to avoid relaxation of the pseudomorphic crystal structure. Unfortunately, the growth rate at such low substrate temperatures is presently quite small, on the order of 50 xc3x85/minute.
The ability to deposit doped epitaxial semiconductor films is a critical technology required in fabricating integrated circuits. It has long been recognized that the ability to deposit semiconductor epitaxial films at reduced substrate temperatures will be required for the fabrication of subsequent generations of integrated circuit devices. Low substrate temperature epitaxial growth of semiconductor films is expected to eliminate or reduce the problems associated with autodoping, dopant diffusion and microcrystalline defect generation in addition to permitting the deposition process to be performed outside of thermal equilibrium. These advantages would result in a reduction in linewidth and junction depth of present device designs, and would permit the fabrication of new device structures currently limited by high processing temperatures.
The ability to grow epitaxial semiconductor films at reduced substrate temperatures using plasma enhanced processes has been demonstrated in numerous investigations. The exact role of the plasma in the deposition process has been attributed to a number of factors, as described in the article by W. J. Varhue, J. L. Rogers, P. S. Andry, E. Adams, M. Lavoie and R. Kontra, entitled xe2x80x9cLow temperature deposition of epitaxial Si,xe2x80x9d Solid State Technology, 163 June (1996). These factors include: production of reactive species which are the precursor to film growth, the removal of adsorbed hydrogen from the growth surface which prevents the adsorption of growth species on the surface, and the increase in adatom surface mobility to lower the required deposition temperature.
Despite the anticipated ability of plasma-enhanced processes to potentially produce hyper-abrupt (1024 atoms/cm4) pn junction structures at lower substrate temperatures, such processes have failed to be successful. Problems associated with the electrical performance of pn junction diodes fabricated by this process have caused concern. The problems are a consequence of interfacial defects that occur at the original wafer surface where the deposition was first initiated.
The defects in the original surface of the substrate are believed to be both chemical and physical in nature and are an artifact of the fabrication process. In the case of the PECVD growth process, the surface has historically been prepared by a combination of ex-situ and in-situ processes. The in-situ process typically involves a plasma-enhanced mechanism which generates either an energetic ion or free radical hydrogen flux onto the substrate surface. It is believed that both of these particle fluxes damage or activate interfacial electrical defects on the wafer surface that then compromise the electrical properties of the diode.
One solution has been to grow a thick epitaxial layer of the same doping composition as the substrate. This effectively removes the substrate surface from the depletion layer of the device. FIG. 1 shows a conventional pn junction diode 320 formed by PECVD and comprising a p-doped substrate 322 and a thick p-doped epitaxial layer 324 deposited onto original surface 326 of substrate 322. After epitaxial layer 324 is deposited, the dopant is changed from p-type to n-type, and thereafter an n-doped layer 328 is deposited. Epitaxial layer 324 allows the metallurgical junction 330 between the oppositely-doped layers 324, 328 to be formed remote from defects present on original surface 326 that interfere with the normal electrical function of diode 320. After the epitaxial layer 324 has been deposited, metal contacts 332, 334 are formed, respectively, on the upper surface of epitaxial layer 324 and the lower surface of substrate 322 to complete diode 320.
Until the present invention, a properly functioning diode could be made using PECVD only by either restricting the doping concentration of the least doped side to 1016 cmxe2x88x923 or by first depositing a film with a doping concentration similar to the substrate. The effect of either of these conditions is to eliminate the possibility of trap-assisted tunneling. In the first case, the depletion region is widened to the point that this leakage mechanism is rendered negligible. In the second case, the effect is to move the metallurgical junction away from the defected initial wafer surface. Growing the additional layer adds a step to the process of forming a pn junction, a step which increases the time, and hence cost, it takes to form the junction and reduces the design flexibility for fabricating device structures.
The present invention is directed to a method of forming an abrupt pn junction on a doped substrate wherein the metallurgical junction between oppositely doped regions may be coincident with the original surface of the substrate. The method includes providing a substrate comprising one of Si and Ge and a first dopant that is either a p-type dopant or an n-type dopant. The substrate has a surface that is substantially de-oxygenated, cleaned of carbon and other impurities, and hydrogen terminated. The substrate is then placed into a reaction chamber and heated to a temperature of less than 650xc2x0 C. A film is then grown by chemical vapor deposition on the surface of the substrate. A second dopant that is either a p-type dopant or an n-type dopant is simultaneously introduced into the chamber while the film is being grown on the substrate.
Another aspect of the present invention is a pn junction diode containing a depletion region and comprising a substrate and a chemical vapor deposited epitaxial layer. The substrate includes one of Si and Ge, has a surface and is doped with a first dopant of either a p-type or an n-type. At least a portion of the chemical vapor deposited epitaxial layer is doped with a second dopant of a type opposite the first dopant such that the surface of the substrate is contained within the depletion region.